This invention relates generally to hydraulic jacks and the like, and more particularly concerns apparatus for halting jack extension at a predetermined location and for preventing the creation of excessive pressure and forces within the jack.
Hydraulic jacks such as those disclosed and claimed in U.S. Pat. Nos. 2,464,254; 2,453,482 and 2,548,903 have met with great commercial success in markets requiring jacks for lifting heavy loads such as truck vehicles and the like. They are compact and safe, yet permit the user to apply relatively great lifting force without excessive effort.
These jacks commonly include a hydraulic cylinder in which is reciprocally mounted a ram. The ram can be hydraulicly extended from the cylinder to provide the desired lifting action. This extension action is provided by pumping hydraulic fluid into a chamber defined inside the cylinder by the cylinder walls, a closed cylinder end, and an opposed ram end.
The introduction of excessive amounts of fluid into this lifting chamber could cause excessive pressure build up within the jack, and consequent damage to the jack by jamming the ram piston end against the cylinder top end. To prevent this, relief ports permit fluid to be bled from the cylinder as the ram piston end reaches a designed maximum extension of its travel. A radially extending relief port is disclosed in U.S. Pat. No. 2,835,541, and another maximum extension relief device is disclosed in U.S. Pat. No. 2,520,426.
To provide the proper seal between the ram piston end and the hydraulic cylinder interior surface, packing devices are provided. Usually this packing includes a sealing gasket or cup member. U.S. Pat. Nos. 2,453,482; 2,520,426; 2,548,902; and 2,548,903 remark upon packing cups formed of leather or other flexible material. But commercial practice has practically dictated the manufacture of such cups from leather. While expensive, this leather material has been able to withstand the high pressures generated within the jack during a long service life, and has exhibited relatively great resistance to damage by cutting or scoring when the cup is moved past a relief hole or port bored or otherwise extended radially through the cylinder wall.
It is the general object of the present invention to provide a pressure relief mechanism for a hydraulic jack of the type described which will inhibit cutting or scoring action directed against the packing cup and consequently will permit the widespread use of packing cups formed of elastomeric material.
It is yet another object of the invention to provide such a relief port which can be offered in the finished jack at a relatively low cost, yet which will prove fully reliable in operation.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings. Throughout the drawings, like reference numerals refer to like parts.